I was considering a system that worked kinda like Oblivion (the Elder Scrolls video game) and only really covers advancement and creation - everything is more or less free form, except for combat, which is pretty simple and largely dependent on what's involved (to fit with a free form theme).

Character creation is fast, just assign primary attributes, and choose a speciality. When you advance, you choose perks from a forest (a forest contains perk trees, hence why it's a forest) that's appropriate to what you've been doing for the past few adventures.

I'm considering allowing GMs to invent pretty much everything specific to a setting. The only thing that remains constant are the basic rules.

The heirarchy of perks helps GMs categorize them for their own games and settings.

Avenues are the broadest things of all. There's only about three avenues that apply to nearly every setting (Nature, Technology and Magic), though a GM is free to invent new ones specific to his setting; yet the only way to do so is in remaining eldritch. Ie. Element 0, or Phlebotinum - both are neither natural, technological nor magical. Otherwise, every setting considers at least some things to be natural and others technological. Many settings also consider certain things magical. Note that this consideration is meta-arbitrary and official, as you might have npcs considering the firearm a kind of magic, for example.

Categories are a bit less broad. Most settings have at least several. Inventing new categories for a given setting is much easier than trying to devise entirely new avenues, while inventing avenues is troublesome and pointless (the same affect can be virtually achieved by inventing a new category). Ie. A category that falls into the Nature avenue might be Physiology or Finesse. A category that falls into the Magic avenue might be psionics, while another would be divine, and another still would be arcane magic.

Disciplines are like the genres of a setting, except that they are sort of like the schools a character might visit to learn their perks. Ie. Telekinesis is a discipline of Psionics, which itself is a category of the Magic avenue. Another discipline might be Light Weapons, which is a discipline of Physiology, which itself is a category of the Nature avenue. Disciplines can be invented by GMs.

Forests are the specifics of a given discipline. Different forests are of different sizes - some may contain relatively very few perks compared to others.

Trees have base perks, needed to be purchased to begin the 'climb'. Some trees are parasite trees (named so because they often depend on investment in perks from other trees first), and have additional entry requirements. When the GM is constructing a tree, it should look a bit like a flowchart with multiple branching paths (simply called 'branches'). Trees are never cyclical, however.

Characters begin with a race, a specialty and some tweaking attributes.

Players can do whatever they like in or out of combat. It’s up to the GM to decide on the associated attributes that should be rolled to determine success. Only one roll per action. If an action would conceivably require multiple attributes, add all of those involved and divide it by the number and then round down. Ie. If vaulting through an open space in time requires Speed, Maneuvering and Precision, then the GM adds each of the modifiers and then divides the sum by 3 to get the modifier to this rolled action - if the modifier is a dice, then the gm divides sides of the dice (ie. 2d6 + 5d6 + 3d6 = 10d6 - divide it by 3 to get 3d6 as the modifier). All actions are rolled as d20 + modifier.

Their are primary and secondary attributes. The primaries are those that can be tweaked at character creation, while the secondaries are tweaked later on.

Primaries determine initial difficulty dice for various tasks that they fall under. The task difficulty is a number - if this number matches the prime attribute the GM decides to associate it with, then the dice challenge is whatever average the player tends to roll. If the associated attribute beats it by at least 1, decrease the dice challenge by 5 for every 1 - if the associated attribute is beaten by at least 1, increase the dice challenge by 5 for every 1.

When prime attributes need to be combined, use the law of averages again, just as you would for combining secondary attributes. Ie. If the two prime attributes needed are strength and dexterity, and the player’s strength and dexterity are 2 and 3 respectively, then the associative attribute is 2 (2 + 3 = 5/2 = 2.5, rounding down to 2).

Prime attributes have no effect on secondary attributes with the exception of determining a dice challenge for a given action they associate with - Ie. Even though most humans with low wound ratings can only suffer 2 light wounds before being disabled, disabled humans with high constitution can retain consciousness and continue fighting when they are more likely to succeed on their dice challenges to do so.

GMs decide on actual difficulties.

Challenge Bonuses
Challenge bonuses are arbitrary modifiers a GM might apply to a challenge in order to scale the capability of the characters. In this way, the actual secondary attributes aren’t arbitrary. Also, a challenge that has a difficulty which meets the player’s associated prime attribute will not use the player’s rolling average as the DC, but something a little higher (or lower, if the GM wants the player to succeed). Finally, it allows the GM to pull a number off the top of their head for assigning a DC instead of trying to think of a difficulty - in this way, it helps them fight optimized characters as well.

Here’s a Guide to Difficulties:
1 - Easy, though some chance of failure.
2:4 - Moderately challenging. Ie. Playing guitar.
5:7 - Difficult. Ie. The math competition requires lots of experience in the subject. Above average natural talent helps.
8:10 - Very difficult. Requires exceptional aptitude. Ie. The most experienced gymnast can’t make the olympics and win gold without at least a naturally high dexterity score, in order to push her training to the next level.
11:13 - Only true heroes are up to the challenge. Ie. Winning gold in olympic level gymnastics has been accomplished hundreds of times. Ever usurped the seat to an empire? That probably hasn’t been done as many times.
14:18 - Nearly impossible/strange reality. Ie. Einstein was way ahead of his time.
19:infinity - Super-human/Godly. Ie. Anything impossible for your species/breaks the laws of physics/etc.

Strength - Anything athletic, or power related. Includes hitting stuff without any actual skill involved. A barbarian in this game is probably almost equally as good with a broken bottle as he is with a sword - only a sword is more practical. Barbarians aren’t trained with swords so much as they are simply willing to end lives on a raw level.

Constitution - Anything relating to endurance. Includes enduring wounds, poison, exhaustion, sickness, stress, etc.
Intelligence - Logical reasoning of internalized systems, memory and retention. Anything involving those. Simple: Bookishness is a personality trait, but bookish people are typically so intelligent that they enjoy their work as a game that may be very tedious and frustrating for others.

Wisdom - Reasoning of open logic, empathy, instinct, creativity and wills. Simple: Detection, judgement, predicting motives, and the power to commit.

Charisma - Personal magnetism and the ability to magnify one’s personality over that of another.

Background

After tweaking prime attributes comes the background. Backgrounds are like feats in D&D, except that they are only chosen at character creation. They determine a character’s early direction. Most characters have one speciality, but characters that are relatively un-heroic might have no specialities. Powerful characters might begin with more than one speciality.

Brawn is pure physical skill, finesse is the use of intelligence and/or physical tricks in combat and Magic is the use of, of course, magic.

Magic is essentially anything strictly non-natural that breaks reality. Magic can be anything with minor exceptions. Sufficiently advanced technology is not necessarily magic, although it may replicate it. Sufficiently advanced technology belongs to avenue of technology - technology can’t be conjured or acquired through character proficiency alone - if it can be, it’s usually with the assistance of other technology and would most likely demand finesse rather than magic. Magitech, on the other hand, is not technology but magic. Using magitech would more likely demand magical specialties or perks (more on perks later). If conjuring technology demands magic, then the entire avenue is magic and the specific category is magitech.

Avenues and categories are quite flexible - what category or avenue something belongs in has no effect on the rules. The GM is even free to invent categories and avenues (for the latter, mostly alien or eldritch concepts specific to a setting). Avenues are already pretty much defined for every concievable setting (them being magic, technology, and nature), but new categories can be conceptualized with some creativity. Ie. Psionics is a category, belonging to the magic avenue. Telekinesis would be a discipline.

If a game begins without more than one speciality, but the GM wants to provide the notion that characters are powerful (the equivalent of a ‘high level adventure’ in D&D), they can allow the players to select perks. Perks are normally acquired throughout an adventure based on what the players have done (ie. a player that’s only been fighting in heavy armor with big swords in every past adventure cannot select perks to do with fighting in light armor or with daggers) and can be made up on the spot, however it’s up to the GM to balance it out.

As a rule, the GM gives the players experience points and places them under forests (a forest contains trees) - the more players focus on a certain forest - that is, have their character perform activities that are appropriate to that forest - the more experience points they attain for that forest. When they have experience points under a forest, they can purchase perks related to the forest. Each perk is part of its own tree - meaning, to purchase a perk, a player must first buy all the perks that fall beneath that given perk. They can also purchase perks that branch away from the given perk. Some perks can also be purchased more than once. GMs are free to invent their own trees and even their own forests!

Perks cover everything, including stuff that’s indirectly related to combat. Normally, experience points are acquired during an adventure or at the end of one, and are spent on perks directly at a cost of 1 per perk, with more powerful perks typically able to be acquired through the tree system.

But since players are going through character creation, they haven’t had the chance to actually test their characters in any adventures yet - thus, there’s no way to determine what perks they’re eligible for!

Because of this, there’s a multiplier to the cost of perks depending on the character’s speciality.

If the player wants to purchase perks from a forest that’s a discipline outside of their character’s speciality or specialities (ie. daggers when their one speciality is called 'swordsman' and exclusively involves swords), then the cost becomes 2 experience points per given perk of that sort. If the perk the player wants is outside of their speciality's (or specialities') entire category, then the cost becomes 3 experience points per given perk of that sort. If the perk is outside the avenue of their speciality, then the cost becomes 5 experience points per given perk of that sort.

Because of this, 'high level' games will typically involve a more specialized assortment of players. Even so, normal games that begin with only specialities for characters and no perks, should naturally make it more difficult for characters performing actions away from their specialities.

Backgrounds should be generic for this reason. A GM should only include a handful (free to invent their own, although these rules will include some samples), and they should exist just to start off an adventure or campaign. There's no advancement paths for specialities - they're essentially just character traits that help out the PCs, although they might sometimes behave like D&D's feats as well.

Some perks can only be attainable by first learning perks from entirely different avenues. For example, spell slinging and waving a sword about may be considered completely different things in a given setting.

Such perks are typically more exotic and unattainable through player creativity alone. If they are a tree, then they're called a parasite tree. In order to access the base of such a tree, a player must first meet its given prerequisites - usually by attaining other perks first. But GMs can allow this to be sidestepped (and prevent character twinking) by allowing a character to attend, ie., a training academy or instruction from a mystical master.

If such perks are merely a branch of a tree, then they simply have additional arbitrary prerequisites before the branch can be scaled.

A setting is called a setting, partly because it sets the rules and constraints. This is why GMs are largely encouraged to at least invent their own perks, trees and forests if they wish to include their own settings. This way, how the perks effect the balance of the game and the success of the characters will coincide with the setting.

Ie. If arcane magic is much harder to cast in heavy armor then in lighter armor (with no armor being optimal), but not impossible, then the GM should make it more difficult for the characters by arbitrarily setting high difficulty bonuses. The GM could then include perks to make it easier, in order to provide direction to players wishing to tailor their characters to spell slinging in heavy armor.

If a GM absolutely doesn't want players to cast spells in heavy armor, with a very minor exception - they can make it part of a parasite tree - a tree that depends on one tree or multiple others (see 'combining disciplines and forests' spoiler for more info).

Avenues and categories are defined in freeform. For trees, it's recommended that the GM construct a model that looks like a flow chart - with the top being the base of the tree and the bottom being the crest.

Backgrounds can be specific or vague. The GM effectively sets the guideline through backgrounds they create.

Barbarian - You are fierce, self sufficient, and tough, proficient with any weapon so far as it allows you to kill something as quickly as possible. Choose two secondary attributes to receive a bonus to - wounds, perception, stamina, or speed. Weapons that you can use are those that take very little time to figure out and/or are commonly known and available. A barbarian can't immediately use guns in a setting with few guns, for example, but could learn later on, through perks. On the especially negative side of things, you aren't 'civilized' meaning you have little or none of the advantages related to dwelling in a city (ie. social skills, academia, ability to read, study habits, how to drive, etc.).

Cat Burglar - You are immediately good at breaking into secure locations, but you aren't good at confrontational fighting.

Academic Mage - You are proficient with arcane magic, but also physically frail. Access arcane magical parasite trees more quickly.

Thug - You are used to getting your way through fighting. You are good at intimidating others, and good with your fists, as well as creating weapons from common items.

Smith - You are able to work in extreme conditions and are sufficiently skilled at working certain metal for certain purposes, enough to ply a trade.

Hunter - You are skilled with some weapons, know the behaviors of certain beasts (player chooses, GM approves), and are a proficient tracker.

Marksman - You are skilled with many ranged weapons and can probably name several models of firearm off the top of your head. You might have a favorite ranged weapon or two, with which you have even greater skill.

Swimmer - You are skilled in most things to do with water activity. You can probably fight better in water too.

The system above allows players to invent their own perks too, provided the GM approves (and then sets them into a tree; although they can let players learn right off the bat too). If players invent a background, the GM should create an associated speciality.

While character creation is simple, leaving the GM to customize the game to the player experience, however, can lengthen the process as the players have to sit and wait for the GM to write and compile things and/or brainstorm with them.